A class of diode pumped alkali vapor lasers (DPAL) demonstrated by Krupke, in U.S. Pat. Nos. 6,643,311 and 7,061,958 B2 requires resonantly matched optical pumps with a spectral width greater than 10 picometers and wavelength corresponding to the D2 transition. The D2 transition occurs between the ground 2S1/2 state and the excited 2P3/2 state. The DPAL is pumped by bleaching to the D2 transition and simultaneously shifting the population of the 2P3/2 excited state to the 2P1/2 state (D1) by spin-orbit mixing. The output wavelength of the DPAL corresponds to the D1 transition.
The natural D2 absorption linewidth is on the order of a few picometers, and the linewidth of most high power diode pump sources had been on the order of tens of thousands of picometers. This mismatch of linewidths did not promote high optical energy conversion efficiency of the pump process. The prior art shows that both increasing the pressure of the alkali vapor and adding selected buffer gases to broaden the D2 spectral line improved energy conversion efficiency. Helium is a common buffer gas, and pressures of several atmospheres have been used. A similar pumping and lasing scheme is known involving metastable rare noble gases having the electronic configuration np5(n+1)s3P2.
To further improve energy conversion efficiency, extensive research in this field has concentrated on reducing the linewidth and locking the wavelength of pump lasers to the D2 transition. Linewidths on the order of 50 picometers or less are currently preferred. Semiconductor lasers with Volume Bragg Gratings (VBG) are often employed for this dual purpose, for example as disclosed in U.S. Pat. No. 7,697,589.
One major disadvantage of VBGs is internal heating caused by the absorption of optical power. Thermal expansion changes the dimensional periodicity of the active grating causing detuning of the wavelength. This effect limits the useful optical power passing through such components to the range of 20-40 W/cm2.
Another disadvantage of VBGs is its wavelength inaccuracy, which must be thermally compensated. Published specifications for the wavelength accuracy of commercial VBG's are typically about ±500 picometers. Because the typical temperature dependence of the laser diode wavelength is about 300 picometers per degree C., the precise resonant wavelength can be thermally tuned by controlling the temperature to within a range of a few degrees C. Strict temperature control (on the order of ±0.05 per degree C.) must then be maintained to stabilize the wavelength.
In addition, VBGs exhibit a dependence of the wavelength of about 7 picometers per degree C. placing further constraints on temperature control.
Birefringent filters (BRF) have also been shown in (U.S. Pat. No. 7,061,958 B2) to promote narrowing of the line width, for example as disclosed in U.S. Pat. No. 7,061,958. However, BRFs require a servo to lock the wavelength, as discussed in U.S. Pat. No. 5,218,610 and references therein. Furthermore, high power BRFs are expensive and very difficult to make.
Faraday Anomalous Dispersion Optical Filters (FADOFs) have been demonstrated to reduce laser linewidth in flash-pumped organic dye lasers to less than 5 picometers and in semiconductor lasers to less than 10 picometers. In addition, a FADOF filter used in combination with a saturated absorption cell was shown to lock the wavelength of a tunable semiconductor external cavity laser diode. All of this prior art utilized a classic FADOF filter configuration with two polarizers.
Additionally, there is a need to reduce the number of optical elements in optical systems to reduce cost and improve transmission efficiencies.